1. Field of the Invention
The present invention relates to an automatic gain control device for a servo loop, an apparatus and method for adjusting the gain of a servo loop of an optical information recording/reproduction apparatus, in particular, an apparatus and method for adjusting the gain of a focusing servo loop and a tracking servo loop, and an optical information recording/reproduction apparatus having an optical head which comprises the control device.
2. Background Art
As focusing and tracking control systems adopted in a conventional optical information recording/reproduction apparatus such as an optical disk, various systems have been proposed. As a popular one of these systems, an astigmatism method and a push-pull method using a four-divided sensor are known. FIG. 1 is a block diagram showing a focusing servo loop and a tracking servo loop based on the astigmatism method and the push-pull method.
Referring to FIG. 1, a four-divided sensor 1 receives light reflected by an optical disk, and a sensor amplifier 2 amplifies outputs from the four sensor portions. The outputs from the sensor amplifiers 2 are analog calculated using adders 3 and 4 and a subtracter 5, thus generating a tracking error signal S1. FIGS. 2A and 2B show signals obtained when an optical head crosses an information track. As shown in FIG. 2A, the tracking error signal S1 has a sine waveform having one track as one period, and 0-level points A1 to A4 in FIG. 2A represent on-track points.
An adder 6 outputs a signal reflecting a total light amount received by the four-divided sensor 1, i.e., a sum signal S2. As shown in FIG. 2B, the sum signal S2 also has a waveform similar to a sine waveform having one track as one period. In general, the sum signal S2 has maximum peaks at on-track (on-land) points and has minimum peaks at on-groove points, and has a sine waveform whose phase is shifted by 90.degree. from that of the tracking error signal S1. Three curves A, B, and C shown in FIGS. 2A and 2B will be described below.
Even in optical disks manufactured by the same manufacturing method, a variation in various parameters cannot be avoided. An information recording/reproduction light beam is reflected by or transmitted through an optical disk, is incident on the sensor, and is converted into a servo signal or an information signal. At this time, if disks have different reflectances (transmittances), signals having different levels are obtained, as shown in FIGS. 2A and 2B. More specifically, the signals S1 and S2 change due to a variation in reflectance of the three disks A, B, and C, as shown in FIGS. 2A and 2B. However, since the levels of the signals S1 and S2 are proportional to the reflectance (transmittance) of each disk, a normalized tracking error signal S3 from which a variation in reflectance (transmittance) is canceled can be obtained by calculating S1/S2 using an AGC (automatic gain control circuit) 11. Even with the same disk, if different optical heads are used, to read the disk the same phenomenon as above is observed due to a variation in transmittance of an optical system or a variation in sensitivity of a sensor. However, a signal obtained in such a case can be converted into a normalized signal by the AGC 11.
The normalized tracking error signal S3 is input to a tracking actuator driver 19 via a gain adjusting circuit 13, a phase compensator 16 and a switch 17 which are arranged to stabilize a servo system, and an adder 18. The driver 19 drives an AT (auto-tracking) actuator 20 on the basis of the tracking error signal S3, thus executing tracking control. In order to correct factors which cannot be corrected by the AGC 11 such as variations in the gain of the AT actuator 20 and the electrical system, and the like, manual or automatic adjustment is performed using the gain adjusting circuit 13. When a light beam is moved to a desired information track, the switch 17 is turned off and a switch 15 is turned on, so that a driving signal from a jump pulse generating circuit 14 is supplied to the AT actuator 20. Thus, a so-called track jump operation for forcibly moving the AT actuator 20 toward the target track is performed.
On the other hand, the focusing error signal based on the astigmatism method is obtained as an output S12 as a result of analog calculations using adders 7 and 8 and a subtracter 9. FIGS. 3A and 3B show signals of the respective units near an in-focus point. The focusing error signal S12 is a sine wave for one period, which has 0 level at an in-focus point A5, as shown in FIG. 3A. On the other hand, a sum signal is obtained as a signal shown in FIG. 3B. Since the levels of these signals change due to a variation in reflectance (transmittance) of disks or a variation in the optical heads, as shown in FIGS. 3A and 3B, a normalized focusing error signal S13 is generated by calculating S12/S2 using an AGC (automatic gain control circuit) 12.
The normalized focusing error signal S13 is input to an AF (auto-focus) actuator 28 via a gain adjusting circuit 21, a phase compensator 24, a switch 25, an adder 26, and an AF actuator driver 27. When the AF actuator 28 is driven based on the focusing error signal S13, focusing control is executed. Upon insertion of a disk, an output signal from an up-down circuit 22 is directly supplied to the AF actuator driver 27 by turning off the switch 25 and turning on a switch 23. Thus, the AF actuator 28 is forcibly pulled up/down to drive an objective lens in the focusing direction, thus performing an AF pull-in operation near an in-focus point. At this time, the focusing error signal changes in an S-pattern, as shown in FIG. 3A, and an AF pull-in is attained by turning off the switch 23 and turning on the switch 25 near the in-focus point A5 of this signal.
The arrangement of the AGC 11 will be described below with reference to FIG. 4. The AGC 11 is a circuit for normalizing the tracking error signal S1 by dividing the tracking error signal S1 by the sum signal S2. A multiplier 30 is a circuit for multiplying two input signals S2 and S8 and outputting an output signal S7. In this case, the multiplier 30 outputs the signal S7=G1.multidot.S2.multidot.S8 (where G1 is the gain constant). An operational amplifier 29 outputs the signal S8 to the multiplier 30, and the signal S8 is negatively fed back to the operational amplifier 29. An offset adjusting volume 39 is set at 0 level, and if the operational amplifier 29 has a sufficiently large open-loop gain at that time, the signal S8 is given by: EQU S8=S1/(G1.multidot.S2) (1)
Therefore, an error signal obtained by normalizing the tracking error signal S1 by the sum signal S2 appears as the output from the operational amplifier 29.
However, in an actual circuit, the signal S8 is influenced by an input offset V.sub.ofst1 of the operational amplifier 29 and an input offset V.sub.ofst2 of the multiplier 30. At this time, an offset V.sub.ofst generated in the signal S8 when S1=0 is given by: EQU V.sub.ofst =V.sub.ofst1 /(G1.multidot.S2)+V.sub.ofst2 ( 2)
The offset V.sub.ofst is represented by a curve shown in FIG. 5. More specifically, of the offset components appearing in the signal S8, the input offset of the operational amplifier 29 is inversely proportional to S2, and the input offset of the multiplier 30 is constant.
Since the sum signal S2 is a signal reflecting the total light amount of light reflected by a disk, when the level of the signal S2 changes from V1 to V2 due to a variation in power of a semiconductor laser or a variation in reflectance of the disk, the offset generated in the signal S8 also changes. For this reason, the offset adjusting volume 39 and an offset adjusting volume 40 are added to allow independent adjustment of V.sub.ofst1 and V.sub.ofs2, and a voltage is forcibly applied from an external circuit to S2 in a state wherein no disk is inserted, thereby changing the signal S2, as shown in FIG. 6A. With this operation, offsets corresponding to V1 and V2 appear in the signal S8. Since a change in output S8 at that time is caused by the input offset of the operational amplifier 29, the volume 39 is adjusted, so that the signal S8 is not changed by S2, as shown in FIG. 6B. In this case, a constant offset which is left unadjusted corresponds to V.sub.ofst2. When this offset is adjusted using the volume 40 to obtain the output S3 having a 0 offset, the offset adjustment of the AGC 11 is completed.
The above description has been given in association with tracking servo and the AGC 11. In the AGC 12, the same offset adjustment as described above can be performed by inputting the focusing error signal S12 in place of the tracking error signal S1.
In general, in an optical disk apparatus, when a light beam irradiates an optical disk, the light beam emitted from a semiconductor laser in a light source is focused by an objective lens, and the focused light beam irradiates the optical disk as a light spot having a spot size of about 1 .mu.m. Such a light spot is controlled to be scanned to track an information track on the optical disk while being focused on the medium surface of the optical disk by feedback control called focusing servo and feedback control called tracking servo. In these feedback control operations, a position error between the light spot and the optical disk surface and a position error between the light spot and the information track are detected on the basis of the output from a sensor for detecting light reflected by the disk, and the objective lens is moved in the focusing and tracking directions in correspondence with these errors, thus attaining the focusing control and the tracking control of the light spot.
However, in the AGC shown in FIG. 4, since the volumes are adjusted while forcibly inputting a voltage to a terminal which should receive the sum signal, adjustment is limited to initial adjustment before a circuit board is delivered. For this reason, a change in offset due to temperature characteristics or aging of an element cannot be adjusted, and focusing servo and tracking servo become unstable. In addition, the adjustment process is complicated.
The control gains of the above-mentioned tracking and focusing control loops are not constant due to variations of the reflectance of a recording disk, the servo signal characteristics of the disk, the sensitivity of an object lens actuator, and the like. For this reason, if the control gains are too large, in the worst case, the control loops oscillate and not only can information not be satisfactorily recorded/reproduced but also recorded data may be destroyed. Thus, a method of automatically adjusting the gains of the control loops has been proposed. However, in order to adjust the control loops, the loop gains must be measured. However, when the loop gains are measured, the control loops become unstable, and the gains cannot be adjusted to required values due to the influence of signals input to the loops, resulting in oscillation of the control loops.